U.S. Pat. Nos. 3,765,872; 3,779,741; and 4,224,057 are exemplary of the type of direct reduction moving bed processes for which the present invention is especially useful.
With the recent dramatic increase in fuel costs, the viability of a commercial process can be seriously jeopardized if it is not fuel efficient. Since catalytic reformers used in direct reduction processes must operate at high temperatures for proper continued function and for producing reducing gas with the proper constituents; such reformers, without any heat recovery, can operate only at about 50% maximum thermal efficiency.
The reformation reaction of light hydrocarbons and naphthas takes place in catalytic-packed tubes at temperatures ranging from 600.degree. to 900.degree. C. The catalyst tubes are typically located in a radiant chamber where they are fire heated. The flue gases produced leave the chamber typically at a high temperature of around 1000.degree. C.
In order to increase the overall efficiency of the reformer, it has been the practice in the past to recover as much of the thermal energy of these flue gases as possible, for example by using heat exchangers to preheat the natural gas-steam reformer feed mixture, to generate the steam necessary for said reaction, and to preheat the combustion air used in the burners of said reformers. By these means it is possible to increase the overall thermal efficiency of the reformers to a range of from 80% to a maximum of 90 or 91%. However, in so increasing the thermal efficiency of the reformer, some of the steam produced is in excess of that required for the reduction process. This is termed "export" steam. This export steam can be used to drive turbines to generate mechanical or electrical energy (for example, for use in driving pumps, compressor motors, and the like). But in many installations, because of the availability of other more cost-effective energy sources, the use of the excess heat from the reformer to produce export steam is undesirable.
It is also a feature of these catalytic reformers that they are very sensitive to thermal shock and should be run at a steady state. For example, start-up of a reformer typically takes about one to three days. As a consequence, it is most desirable that the reformer need not be shut down during short-term processing interruptions.
The other major source of energy consumption in these processes is the heater used to raise the temperature of the de-watered make-up reducing gas and/or recycled reducing gas to a level adequate for the reduction of the ore, namely from 700.degree. to 1100.degree. C., and preferably between 870.degree. to 950.degree. C. Typically the exit temperature of the flue gas from this separate heater is normally kept in the range of 140.degree. C. to 200.degree. C., and preferably about 160.degree. C. The operating conditions of the heater depend upon the particular operating conditions at the reactor which may vary, for example, due to change in productivity or in the type of iron ore charged.
It is an object of the present invention to provide a method and apparatus for reducing metal ores to metal particles with less fuel overall than was formerly required.
It is a further object of the present invention to provide such method and apparatus with an improved overall thermal efficiency.
It is a still further object of the present invention to achieve the foregoing objects in a more efficient and economical manner than was heretofore possible by the former processes.
It is yet a further object of the present invention to provide a method and apparatus which affords greater flexibility in overall plant design and operation.
Other objects and advantages of this invention will become clear from the following description of the invention and its preferred embodiments.